The volume contains selected contributions from the scientific programme of the 5th International Meeting of Young Computer Scientists (IMYCS '88) held at Smolenice Castle (Czechoslovakia), November 14-18, 1988. It is divided into five chapters which approach the three crucial notions of contemporary theoretical computer science - machines, languages, and complexity - from different perspectives. The first chapter contains contributions dealing with problems of decidability, hierarchy, and complexity. Papers concerning different types and problems of automata theory form the second chapter. The contributions in the third chapter cover the large field of algorithmics from the study of program complexity to the domain of computational geometry. The two contributions of the fourth chapter are devoted to logic programming and inductive inference. The final chapter deals with problems of cryptography and contains the text of the IMYCS '88 tutorial on cryptography and data security delivered by A. Salomaa. The book will be a useful source for orientation in contemporary theoretical computer science and related fields such as software engineering and artificial intelligence for researchers and graduate students.